Rat factor X is synthesized as a single chain precursor inducible by prothrombin fragments

Factor X in plasma is a gamma-carboxylated two-chain glycoprotein which, in activated form, plays a pivotal role in blood coagulation. We have utilized purified rat Factor X antibody, coupled to Sepharose, to isolate and characterize Factor X in rat liver, plasma, and hepatoma cells. Rat factor X is synthesized as a single chain precursor (Mr = 63,000). It is this form which undergoes vitamin K-dependent carboxylation in rat liver microsomes. Only after secretion is Factor X converted into its two-chain mature form. Single chain X synthesis and secretion in hepatoma cells is enhanced by vitamin K. The amount of single chain X secreted by these cells is one-half that of prothrombin. The NH2-terminal gamma-carboxylated fragments of prothrombin which induce prothrombin synthesis (Graves, C. B., Munns, T. W., Carlisle, T. L., Grant, G. A., and Strauss, A. W. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 4772-4776) also induce single chain X synthesis by hepatoma cells. We propose that synthesis of all vitamin K-dependent proteins may be regulated by this common control mechanism.